backstay

From back +‎ stay.

backstay (plural backstays)

2. (nautical) A part of the rigging of a sailing ship extending from the top of the mast (masthead) to the back of the ship; they support the strain on all upper masts and provide additional support to the shrouds when the wind is abaft the beam.
3. A strengthening or supporting piece that is built into the back of something.

• forestay

• stay back